1. Field of the Invention
The present invention relates to an ion implantation apparatus. More particularly, the present invention relates to a Faraday assembly of an ion implantation apparatus, for moving a Faraday cup within a vacuum chamber of the apparatus.
2. Description of the Related Art
Generally, ion implantation apparatus have been used for a long time in the manufacturing of semiconductor devices. In particular, an ion implantation apparatus forcibly implants ions into a selected region of a wafer made of silicon, for example. The ion implantation apparatus includes a particle accelerator to accelerate the ions as a beam towards the wafer. The ions collide with the silicon atoms of the wafer, thereby gradually losing energy, and stop at a certain depth in the crystal lattice structure of the silicon.
The ion implantation apparatus also includes a Faraday cup. The Faraday cup functions as a sensor for measuring the current, energy, and shape of the ion beam as well as the absorption of ion beam energy at the end of the ion beam. The Faraday cup is typically located in front of a target (the wafer) in a vacuum chamber sometimes referred to as the “target chamber”. The location of the Faraday cup in the vacuum chamber can be adjusted according to the region of the target selected for ion implantation. The apparatus for changing the location of the Faraday cup is referred to as a Faraday assembly. The Faraday assembly thus includes a Faraday cup located in a vacuum chamber and power means for moving the Faraday cup in the vacuum chamber in the direction of propagation of the ion beam.
The power means includes a reversible motor, and a ball screw connected to the motor for converting the rotary output of the motor into rectilinear motion. Accordingly, the ball screw can be rotated in forward and reverse directions according to the rotary direction of the output of the motor, so that the Faraday cup can be moved rectilinearly back and forth within the vacuum chamber.
The interior of the vacuum chamber is maintained at a low vacuum pressure of about 10−3 torr to minimize contamination of the wafer during the ion implantation process. On the contrary, the environment outside the vacuum chamber is at atmospheric pressure. Accordingly, it is relatively easy to advance the Faraday cup within the vacuum chamber. However, a relatively large load is applied to the power means by the vacuum in the vacuum chamber when the Faraday cup is retracted within the vacuum chamber. In this case, the motor is overloaded and draws excessive current, whereupon the Faraday cup is positioned at an incorrect location within the vacuum chamber.